Human Papillomavirus (HPV), a non-enveloped, deoxyribonucleic acid (DNA) virus, belongs to the family Papillomaviridae. The viral genome is a double-stranded, closed circular DNA, which is approximately 7.2-8 kb in length and contains 8 open reading frames (ORFs). The genome can be divided into three parts in terms of function: (1) the early region (E), approximately 4.5 Kb in length, coding for 6 non-structural proteins E1, E2, E4-E7 associated with virus replication, transcription and transformation; (2) the late region (L), approximately 2.5 Kb in length, coding for the major capsid protein L1 and the minor capsid protein L2; (3) the long control region (LCR), located between the end of the L region and the initiating terminal of the E region, approximately 800-900 bp in length, and comprising regulator elements for DNA replication and expression instead of coding for proteins. HPV viral particles have a diameter of 45-55 nm, wherein the nucleocapsid, consisting of L1 and L2, exhibits icosahedral symmetry and comprises 72 capsomers.
Currently, there are over 100 different types of HPV, mainly causing papillary disease in the skin and mucosa of human. HPV types are divided into three groups depending on their relation with tumorigenesis: (1) group of low or no cancerogenic risk, containing HPV 6, 11, 39, 41, 42, and 43; (2) group of medium cancerogenic risk, containing HPV 31, 33, 35 and 51; and (3) group of high cancerogenic risk, containing HPV 16, 18, 58, 45 and 52.
HPV molecular epidemiological investigation demonstrates that infection by high-risk HPV types is an important factor responsible for the development of cervical cancer. Among all the cervical cancer specimens, HPV DNA is detected in over 80% of them. Cervical cancer is a common malignant tumor among women, the incidence of which is only next to breast cancer, and seriously threatens the health of women. There are about 490,000 newly reported cases worldwide every year, and nearly 270,000 people die of this disease annually (Boyle, P., and J. Ferlay. Ann Oncol 2005, 16:481-8). Cases in developing countries account for approximately 83% of the total cervical cancer cases. In these developing countries, the cervical cancer cases account for about 15% of female malignant tumors, in contrast to 1.5% in developed countries. Cervical cancer is most prevalent in sub-Saharan Africa, central and Southern Asia, Latin America, and Eastern Asia. Cervical cancer is also prevalent in China. The incidence of cervical cancer among married women is as high as 1026/100000 in Lueyang County of Shanxi Province.
The distribution of HPV types exhibits some characteristics of geographical distribution and populations. HPV 16 and 18 subtypes are the most common types in cervical cancer worldwide, and HPV52 subtype is the sixth most common high-risk HPV type. Among some areas of China, for example, provinces such as Guangdong province, HPV52 is a high-risk cancerogenic HPV type only next to HPV 16, 33 and 18.
Currently, the commercially available HPV vaccines are Gardasil® from Merck and Cervarix® from GSK, which comprise HPV6/11/16/18 and HPV16/18 VLP, respectively, but do not comprise HPV type 52 VLP.
Therefore, vaccines directed to HPV type 52 shall be involved in the development of vaccines for high-risk types, which cover a wider scope and are more suitable for Chinese population.
HPV L1 protein, with a molecular weight of 55-60 kDa, is the major capsid protein of the human papillomavirus and the main target protein of the HPV vaccine. HPV L1 protein expressed in many expression systems can form Virus-Like Particles (VLPs) which resemble native HPV particles morphologically, without the assistance of the L2 protein. The VLPs, consisting of 72 pentamers of the L1 proteins, exhibit icosahedral symmetry. Since the VLPs retain the native epitopes of the viral particles, they are highly immunogenic and can induce the generation of neutralization antibodies against homologous HPV (Kirnbauer, R., F. Booy, et al. 1992 Proc Natl Acad Sci USA 89(24): 12180-4). Furthermore, the VLPs are safe and have no potential cancergenic risk as they contain no viral nucleic acids. Therefore, VLP vaccines have become the primary candidate for HPV vaccines.
The key for development of HPV VLP vaccines lies in efficient production of VLP samples in large-scale. Currently, the most common expression systems used for VLP are divided into eukaryotic expression systems and prokaryotic expression systems.
The commonly used eukaryotic expression systems comprise poxvirus, insect baculovirus and yeast expression systems. HPV L1 protein expressed in eukaryotic expression systems shows little conformational difference from that of the native virus, and can self-assemble into VLPs. Thus, purified VLPs can be easily obtained after simple gradient density centrifugation. It brings a lot of convenience to the purification work. However, due to the high culture costs and low expression level of eukaryotic expression systems, it is quite difficult to product industrially on a large-scale. The HPV vaccine Gardasil®, which came into the market recently, is more expensive than others due to low expression level and high production cost of the Saccharomyces cerevisiae expression system employed in its manufacture, and therefore, its general application is limited.
The expression of HPV L1 protein in a prokaryotic expression system such as E. coli expression system has been previously reported. The expression of HPV 16 L1 protein by employing E. coli has been reported (Banks, L., G. Matlashewski, et al. (1987). J Gen Virol 68 (Pt 12): 3081-9). However, most HPV L1 proteins expressed in E. coli lose their native conformation and cannot induce protective antibodies against HPV. Alternatively, although HPV VLPs can be obtained from the proteins by steps such as purification from inclusion bodies and renaturation (Kelsall, S. R. and J. K. Kulski (1995). J Virol Methods 53(1): 75-90), it is difficult to apply this method to large-scale production, as the proteins are largely lost during the renaturation process and the yield is low. Although HPV L1 protein may be expressed in a soluble form with a correct conformation in E. coli and be dissolved in the supernatants of E. coli lysate, the expression level is low. Moreover, since there are large number and amounts of impure proteins, it is difficult to isolate the proteins of interest from them. Although it is also reported that the expression level of L1 protein can be increased in the supernatants by means of GST fusion expression and the purification of the protein of interest is facilitated (Li, M., T. P. Cripe, et al. (1997), J Virol 71(4): 2988-95), it still cannot be applied to larger-scale production because expensive enzymes are required to cleave the fusion protein.
Therefore, the obtainment of a HPV L1 protein capable of inducing the generation of protective antibodies against HPV, and a virus-like particle consisting of the same, at low cost, are still urgent in the art, in order to make the large-scale industrial production of vaccines for cervical cancer possible.